<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <link rel="stylesheet" href="./css/all.css">
</head>
<style>
    .fas .inner{
        width: 200px;
        height: 0px;
        display: none;
        background-color: blueviolet;
        transition: height 3s;
    }
    .fas:hover .inner{
        display: block;
        height: 200px;
        
    }
</style>
<body>
    <div class="fas fa-bell">
        <div class="inner">

        </div>
    </div>
</body>
</html>